This invention relates to a transportable apparatus for drying the stator windings of a generator in preparation for water leak tests.
The water cooled stator windings of large generators are required to be tested at regular intervals to detect any water leaks in the winding by vacuum and pressure decay tests, and to determine the integrity of the insulation system by electrical tests. For both these tests, the water in the winding must be drained and the water passages in the winding dried completely, free of any traces of moisture. This drying and leak test process is currently accomplished with the use of a Hydraulic Integrity Test (HIT) Skid that contains the various apparatus required to perform the test.
In preparation for the test, water retained in the stator winding after the stator winding cooling water system is shut down is drained by opening the drain valves in the inlet and outlet headers. The remaining bulk water is removed by blowing compressed air through the winding. The stator winding is then pressurized with dry compressed air and the air is then released at a rapid rate using a quick release valve. This pressurization and quick decompression, or xe2x80x9cblow downxe2x80x9d cycle is repeated until no visible mist is present in the air being discharged from the winding. A vacuum is then applied to the winding to boil off remaining traces of moisture until a high level of vacuum is achieved and retained in the winding. Vacuum and pressure decay tests are then performed on the winding.
Currently, the average time required to complete the drying operation and the pressure and vacuum decay tests is about four days. As work on the generator is often in a critical path in the schedule for a turbine generator overhaul, utility customers seek to reduce the time required for these tests even further, and development of this invention addresses that objective.
The portable test skid in accordance with the invention contains all necessary equipment to perform the HIT. The major components include a microprocessor controlled compressor, an air dryer, a buffer tank, a main receiving tank and a vacuum pump. In addition, the skid contains all necessary power and control leads, hoses, flanges and valves to hook the skid up to the generator, along with control devices (i.e., microprocessors) and a control panel. The skid is thus a self-contained unit that is transportable to the customer utility site for the stator winding testing scheme.
The compressor is arranged to deliver heated air to the main receiving tank. The compressor has a greater HP rating than compressors used on prior skids. Upstream of the receiving tank, the compressed air flows through multi-channel dryer towers to dry the air and thus enable the air to carry more moisture out of the stator winding. Between the dryer towers and the main receiving tank, a buffer tank is provided that fills with compressed air as the receiving tank empties to the stator winding. This arrangement allows the compressor to operate continuously and thus reduces time between blow down cycles. It is also a feature of the invention to include larger heaters within the main receiving tank to increase the temperature of the compressed air supplied to the winding. Raising the temperature of the air further enhances the ability of the air to carry moisture and thus contributes to the reduction in time required to complete the blow down cycles.
After the moisture in the stator winding has been removed to a satisfactory level, the vacuum and pressure tests are carried out to detect any water leaks in the stator windings.
The use of a compressor with high horsepower and output capacity, coupled with the incorporation of a buffer tank on the skid, and the higher capacity heater in the receiving tank, reduces the drying and testing time by about 34% as compared to prior HIT skids.
Accordingly, in one aspect, the invention provides a transportable skid for drying and testing generator stator windings comprising a platform supporting a compressor adapted to supply compressed air to the stator winding; a dryer arranged to receive and dry compressed air from the compressor; a buffer tank arranged to receive a relatively small portion of compressed air from the compressor after passing through the dryer; and a receiving tank arranged to receive compressed air from the buffer tank and the compressor after passing through the dryer.
In another aspect, the invention relates to a method of drying a stator winding comprising a) providing on a transportable skid a compressor adapted to supply compressed air to the stator windings; a dryer arranged to receive and dry air from the compressor; a buffer tank arranged to receive compressed air from the compressor after passing through the dryer; and a receiving tank arranged to receive compressed air from the compressor after passing through the dryer; b) supplying compressed air from the compressor to a receiving tank at a first predetermined temperature and pressure; c) releasing the compressed air in the receiving tank to the stator winding; d) supplying compressed air to the buffer tank during step b); e) after step b), supplying compressed air from the buffer tank to the receiving tank to partially fill the receiving tank; f) supplying compressed air from the compressor to the receiving tank to completely fill the receiving tank; and g) repeating steps b) through e) until the stator winding is dry.